Relatively small and portable tapered thread cutting machines finding utility in the cutting of tapered threads on the ends of bar stock such as the steel used to reinforce concrete are currently being utilized by Erico Products, Inc. of Solon, Ohio and Erico Europa B.V. of Tilburg, Holland in connection with the coupling of concrete reinforcing bars known as the LENTON system. LENTON is a trademark of Erico Products, Inc. for both the machines for forming the tapered threads and for the coupling system.
In such machines a rotary machining head is employed which includes a recess in its face. Within the recess which is in the form of a regular polygon or square, tooling holders are mounted which in turn support the chasers which may be opened and closed for successive passes for the formation of the thread. A linear cam through a thrust block causes movement of one of the chaser holders which in turn because of their cooperating configuration within the recess causes movement of all.
Because of the interaction of the tooling holder with the inside surface of the recess as well as a back wear plate, the surfaces thereof must be machined and finished to a substantial degree of precision to avoid binding or chattering during the movement of the parts and to prevent excessive wear.
The rotary machining head is normally formed from a single large casting with the recess being formed therein by an end mill operation. After the recess is formed the back wear plate is normally press fit into the recess thus formed. Because of the end mill operation which necessarily entails tooling or spindle deflection, and the press fit operation, the precision of formation of the inside surfaces of the recess and the wear plate, when assembled within the recess, cannot be ensured. Furthermore, the balance of the rotary machining head which is connected to the transmission is unnessarily massive and heavy when utilizing a one-piece casting.
Further contributing to binding is the employment of the cam thrust block which is fastened to one of the holders and which is held against the linear cam by a spring which seats against the back of the thrust block. This type of design may tend to create an eccentic loading which is tranferred from the thrust block to the associated tool holder further creating binding in the movement of the tool holders which causes problems in both the opening and closing, and particularly closing of such holders.
Furthermore, prior art tool holders and the chasers supported thereby are unnessarily complex with the chasers fitting in recesses or openings in the tool holders and requiring disassembly of the tool holders from the recess to replace the chasers. Moreover, the special chasers required to fit the tool holders are of complex shapes requiring a multiplicity of machining and grinding operations making the wear replacement chasers unduly expensive to manufacture in addition to the problem of disassembly for replacement.
Reference may be had to Canadian Pat. Nos. 1,101,617 and 1,108,357 for an illustration of some types of prior art threading machines.